The present invention relates to the art of steam pressure regulation. More particularly, the invention relates to a method and apparatus for controlling steam flow through heated cylinders. The present invention finds particular application in conjunction with drying cylinders for paper making machinery and will be described with particular reference thereto. It is to be appreciated, however, that the invention is also applicable to other steam and condensable vapor heated structures.
In paper making machinery, the paper products are passed over a series of drying cylinders or drums. The drying cylinders are commonly heated by passing selected amounts of steam thereinto where the steam condenses into water condensate releasing its heat to the cylinder. During the removal of the condensate from the cylinder, some steam is also removed. Various systems have been developed for recovering and minimizing the amount of heat which is lost in steam removed with the condensate.
One system of controlling heat loss is disclosed in U.S. Pat. No. 4,222,178, issued September, 1980 to T. L. Moran. The Moran system seeks to maintain a preselected ratio between the flow rate of the condensate and the flow rate of the removed steam. Specifically, a controller compares the condensate and removed steam flow rate ratio with a preselected ratio for the current operating conditions, and controls an atmospheric relief valve in such a manner that the monitored ratio converges upon the selected ratio.
Others have monitored the temperature of the removed steam and condensate and utilized that monitored temperature to control the amount of steam fed to the drying cylinder. Also, others have suggested controlling the pressure differential between the inlet and the outlet of the drying cylinder in accordance with the amount or rate of the paper passing over the cylinder. Still others have adjusted the pressure of the removed steam as a function of the temperature of the removed steam and condensate.
The prior art control systems have tended to be relatively complex. Monitoring the flow rate of steam, for example, requires apparatus which is relatively expensive, yet relatively inaccurate. Further, steam flow measuring apparatus costs energy by creating a pressure loss.
The present invention contemplates a new and improved steam pressure control system for drying cylinders and the like which overcome the above referenced problems and others, yet maximizes the efficiency of steam usage.